1. Field of the Invention
This invention relates to high density information record media of an electrostatic capacitance type, such as video or digital audio discs, on which an information signal is recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems of the electrostatic capacitance type, an information signal is recorded as geometric variations by forming pits in a spiral plane or groove or in concentric planes or grooves, thereby obtaining high density information records such as video or digital audio discs. The individual pits of the record are very small and are arranged in a very high density. When the geometric variations are traced with a pickup stylus such as a diamond having an electrode, capacitive variations are established between the stylus and the record according to the geometric variations, so that the recorded information signal can be played back or reproduced.
Several kinds of information signal records of the electrostatic capacitance type have been proposed for use in the above type of electrostatic capacitance playback system. In one such record medium, an electrode is provided on one or both surfaces of a record substrate on which geometric variations have been press-molded according to an informatio signal. This permits formation of an electrostatic capacitance to be established between the record electrode and an electrode of pickup stylus during the course of reproduction operations. More particularly, the record substrate, on which pits are formed as desired, is covered with a metallic thin film several hundred angstrom in thickness, and also with several hundred angstrom thick dielectric layer overlaying the metallic film. The dielectric layer serves to prevent short circuiting of the electrodes and increases the dielectric constant between the electrodes. However, the record of this type needs a number of fabricating step including a press molding of the record substrate, depositing the metallic thin film and the dielectric layer, and the like. Thus, the fabrication is complicated and troublesome with the need of a relatively large-scale manufacturing apparatus. Thus, the production cost becomes very high.
Another type of information record medium is known. This medium is obtained by press-molding conductive plastic compositions comprising polyvinyl chloride resins, lubricants and several tens percent of carbon black so that an information signal is recorded as geometric variations. In this type of recording medium, an electrostatic capacitance is established between the electrode of a pickup stylus and the record itself. Thus, the step of depositing a metallic thin film on the substrate is not necessary. In addition, because fine particles of carbon black are individually covered with the resin, the dielectric film is not necessary as well. Thus, this type of record medium can be simply manufactured at a relatively low cost.
However, the record medium has a serious problem resulting from carbon black which is added to vinyl chloride resins in large amounts sufficient to impart a relatively high conductivity to the resin composition. In general, carbon black contains large amounts of impurities such as ashes. This is considering to be inevitable in view of the production process of carbon black. During or after the molding of the record medium such as video disc, the impurities contained in carbon black appear on the surface of the medium and render the medium surface irregular. The irregular surface gives an adverse influence on playback characteristics, e.g. dropout defects will be produced.
In order to solve the above problem, we proposed, in Japanese Laid-Open Patent Application Nos. 58-56243, 59-191153 and 59-215041, information signal recording media which made use of conductive carbon black which were washed with water or an acid to remove the impurities thereform. However, if carbon black is washed, its surface is activated, causing the pH of carbon black to lower. This will promote the thermal decomposition of vinyl chloride resins on heating during the molding operation.
Another problem involved in the recording medium of the electrostatic capacitance type is the wear of the record medium and the playback stylus by contact therebetween. The wear of the record medium can be mitigated by addition of a silicone oil such as dimethylpolysiloxane. However, the silicone oil tends to bleed out during the operation of press molding, so that pits cannot be formed exactly and the medium surface is undesirably contaminated with the oil. In addition, the use of the silicone oil is not so effective in preventing the wear of the playback stylus with an attendant problem that the electrode is separated from the stylus.
As a result of the investigation on these prior art problems, we found that the moldability and wear resistance of the record medium were greatly influenced by the surface characteristics of a conductive material, such as carbon black.
The record media of the type which is formed from a conductive vinyl chloride resin composition generally comprise a large amount of a conductive material. The vinyl chloride resin used serves as a binder for the particles of conductive material, i.e., the resin is considered to cover the individual particles.
The playback stylus usually moves at a speed of about 10 meters/second under a load of 400 to 500 kg/cm.sup.2. Although the vinyl chloride resin may be imparted with lubricating properties by addition of suitable lubricants, the record medium will become defective in portions where the particles of a conductive material are exposed. These portions are considered to contribute to the shortage of life of the playback stylus and the medium per se.
As described before, the use of a lubricant such as dimethylpolysiloxane in large amounts undesirably brings about the bleedout at the tie of the press molding and causes the molding failure of pits, leading to the lowering of productivity.